The invention is based on a nonlinear resistor with varistor behaviour according to the preamble of Patent Claim 1. This resistor contains a matrix and a filler in powder form which is embedded in the matrix. The filler contains sintered varistor granules with predominantly spherical particles of doped metal oxide. The particles are made up of crystalline grains separated from one another by grain boundaries. Since, compared with comparably effective resistors based on sintered ceramic, the elaborate sintering processes can be made substantially simpler, composite resistors of this type can be produced relatively straightforwardly and in a larger variety of shapes. The invention also relates at the same time to a method for the production of this resistor.
A resistor of the type mentioned above is described in R. Strxc3xcmpler, P. Kluge-Weiss and F. Greuter xe2x80x9cSmart Varistor Compositesxe2x80x9d, Proceedings of the 8th CIMTECH World Ceramic Congress and Forum on New Materials, Symposium VI (Florence, Jun. 29-Jul. 4, 1994). This resistor consists of a polymer filled with a powder. As the powder, use is made of granules which have been produced by sintering a spray-dried varistor powder based on a zinc oxide doped with oxides of Bi, Sb, Mn, Co, Al and/or other metals. These granules are composed of spherical particles, shaped like a football, which have varistor behaviour and are made up of crystalline grains separated from one another by grain boundaries. The diameters of these particles are up to 300 xcexcm. By varying the dopants and the sintering conditions, the electrical properties of the sintered granules, such as nonlinearity coefficient xcex1B and the breakdown field strength UB[V/mm], can be adjusted over a large range. With the same starting materials, a resistor of this type has a higher nonlinearity coefficient and a higher breakdown field strength if the proportion of filler decreases. It has, however, been shown that then, when limiting a voltage, the acceptance capacity for energy is relatively low.
WO 97/26693 describes a composite material based on a polymer matrix and a powder embedded in this matrix. As the powder, granules are used which have likewise been produced by sintering a spray-dried varistor powder based on a zinc oxide doped with oxides of Bi, Sb, Mn, Co, Al and/or other metals. These granules have spherical particles formed in the shape of a football which have varistor behaviour and are made up of crystalline grains separated from one another by grain boundaries. The particles have diameters of at most 125 xcexcm and have a size distribution which follows a Gaussian distribution. This material is used in cable connections and cable terminations, in which it forms voltage-controlling layers.
U.S. Pat. Nos. 4,726,991, 4,992,333, 5,068,634 and 5,294,374 disclose voltage-limiting resistors made of a polymer and a filler in powder form based on conducting and semiconducting particles. In these resistors, the overvoltage protection is achieved by dielectric breakdown of the polymer. Since relatively high temperatures can occur in this case, the overvoltage protection ought not to be reversible and the energy acceptance capacity ought to be relatively low.
The object of the invention, as specified in the patent claims, is to provide a resistor which, despite having a high nonlinearity coefficient for a good protection characteristic, is distinguished by a high power acceptance, and at the same time to provide a method with which a resistor of this type can be produced in a particularly advantageous manner.
Through a selection of suitable filler, in the resistor according to the invention electrical properties are achieved which come relatively close to a varistor based on a ceramic. In this case it is essential for either a suitably structured conductive additive filler to be provided and/or for varistor granules to be used which permit a particularly high packing density. It is then possible, using a technique known from injection moulding, extrusion or casting resin technology, to produce resistors in a relatively straightforward way which have varistor behaviour and are distinguished by a good protection characteristic and a high power acceptance. It is particularly advantageous in this case that, through suitable selection of the starting components and through readily adjustable process parameters, it is possible to produce varistors which, in terms of their shaping and their physical properties, have a greatly diversified spectrum and, in particular, a relatively high energy acceptance or switching capacity.
The nonlinear resistor according to the invention may advantageously be used as a field-controlling element in cable fittings or as an overvoltage protection element (varistor). It can be used both in low and in medium and high voltage engineering and, because of its simple production and processability, may without difficulty have a complex geometry. If appropriate, it can, for example as a protection and/or control element, be integrated directly by overmoulding on an electrical device, for example a power circuit breaker, or be applied as a thin coating. It may also be used in screen printing in the hybrid process for integrated circuits.
In the process according to the invention, electrically conductive particles also provided in the filler in addition to the varistor particles are bound to the varistor particles on their surfaces before the filler and matrix material are combined. During the combination, the electrically conductive particles can be relied on not to detach from the surfaces of the varistor particles, so that resistors produced using this process have outstanding electrical properties, and in particular extremely stable current/voltage characteristics.
Particularly good electrical properties are achieved if the loose electrically conductive particles that are still present are removed from the filler, for example by washing, screening or air separation, prior to the combination with the matrix material which is primarily carried out by mixing an infiltration.
A further effect achieved by the process according to the invention is that the electrically conductive particles are distributed uniformly over the surfaces of the varistor particles and enter into atomic bonding with the varistor material. The contact action of the filler is thus very substantially improved and a comparatively small proportion of electrically conductive particles in the filler is sufficient for obtaining resistor with outstanding electrical properties, such as in particular a high current-carrying capacity.